A battery feed circuit supplies a dc feed current to telecommunications equipment through a 2-wire transmission path that also typically conducts a bidirectional voice or data signal. This feed current is balanced, i.e., each transmission wire conducts a current that is equal in magnitude but opposite in direction to the other.
In telecommunications applications, the length of the 2-wire transmission path varies considerably. As a result, the battery feed circuit must be designed to supply the requisite dc feed current over a range of path resistances. Transmission paths are also susceptible to induced longitudinal or common mode signals, e.g., a 60 Hz power signal, which can be transformed into noise. It is, therefore, necessary that the battery feed circuit minimize the transformation of such longitudinal signals into noise.
A variety of battery feed circuits have been developed. In general, these circuits can be grouped into one of two classes depending on their battery feed profile, i.e., relationship of dc feed current versus voltage across the 2-wire transmission path, and the manner in which they deal with longitudinal signals. The first class of battery feed circuits (see, for example, U.S. Pat. No. 4,004,109 to F. S. Boxall, issued Jan. 18, 1977) produces a linear battery feed profile and has a low common mode impedance to longitudinal signals. The noise induced in the 2-wire transmission by the longitudinal signals is minimized by this first class of circuits with only a moderate degree of success. Moreover, while such circuits perform satisfactorily in many applications, the use of a linear battery feed profile dissipates excessive power on short 2-wire transmission paths. The second class of battery feed circuits, such as disclosed in a publication entitled "A Floating Low-Power Subscriber Line Interface" by L. Freimanis and D. P. Smith, ISSCC Digest, 1980, page 180, 181, provides a non-linear battery feed profile which limits feed current on short 2-wire transmission paths. The noise induced by longitudinal signals is minimized by providing a high common mode impedance through the use of isolation devices, such as transformers or opto-isolators. These devices, however, are expensive and bulky. Therefore, existing battery feed circuits having a non-linear feed profile are not amenable to size reduction using integrated circuit technology.
In view of the foregoing, development of a low cost integrable battery feed circuit having a non-linear feed profile and reduced susceptibility to longitudinally induced noise would be beneficial.